Re: That's in in a nut shell
"Their is no reason, good or bad, why the GUI needs to be part of the operating system."
The problem is that MS need to create some element of newness and consumer sparkle to the OS every few years (hence why W10 will not be the last version of Windows) to avoid becoming just another IT company.
However, what you suggest for the separation of OS from GUI, is exactly what business would like as it creates stability, and enables innovation on this platform, like we saw with XP and 2003...